The present invention relates generally to utilization of x-ray fluorescence in a photo detection system. More particularly, the present invention provides an improved photo detection system by utilizing various inorganic phosphors that respond efficiently to x-ray excitation in order to produce visible emission more effectively than now provided with ultraviolet excitation.
It is known to employ various ultraviolet radiation excitable phosphors to detect objects marked with said phosphors by optical photo detection means. For example, in U.S. Pat. No. 3,881,941, assigned to the present assignee, there is disclosed a utilization of alkali metal polytungstate phosphors containing europium for an automated postal sorting system wherein the postal stamps or imprints contained upon envelopes, post cards, magazines, packages and the like that bear a marking ink containing said phosphor are identified with a photo detection system. By using such identification means, it becomes possible to distinguish any stamps bearing said marking ink and route the mail automatically after detecting the information therein provided such as: cancellation, local delivery, air-mail delivery, etc. With said photo detection system, the mail is delivered automatically by conveyance means to a detection system including an ultraviolet radiation source positioned to irradiate mail marked with said ultraviolet radiation excitable phosphor and which further includes optical photo detection means positioned adjacent said ultraviolet radiation source which respond to the visible emission produced by the irradiated phosphors. As disclosed in said prior art patent, such photo detection system can be made responsive to phosphor afterglow as a means to avoid problems experienced when background luminescence is also produced by pigments and dyes found in the mail envelopes or other packaging materials marked with this type detection ink. While pulsing of the ultraviolet radiation source is also mentioned as a further means to avoid false detection signals generated by background fluorescence along with elaborate optical filtering of the visible emission given off by the irradiated phosphor, the organic and inorganic phosphors now being used in this manner do not provide a strong fluorescent signal. A greater intensity of the fluorescent signal with respect to the background luminescent signal is thereby still needed to provide a better "signal to noise ratio" for response by the optical photo detection means being employed in this type detection system.
In more recently issued U.S. Pat. No. 3,995,157, also assigned to the present assignee, there is disclosed a surface flaw detection system which employs particular organic phosphor materials suspended in a coating composition with an organic binder. There is again employed an ultraviolet radiation source to produce visible emission in the phosphor coating and with said visible emission being detected by optical photo detection means to reveal flaws when reflected from the coated surface of the object being inspected. It is also recognized in said patent that visible illumination can interfere with optical detection of the flaws when reflected from the treated surface so that particular optical photo detection means are selected for a better signal to noise ratio.